Windows App Certification Kit - cannot install package because a different package with the same name is already installed

When running the ‘Windows App Ceritification Kit’, you get an error message like this:

Windows cannot install package 780a2c2f-f4be-4a7d-83bf-212522026da9_1.0.0.0_neutral_~_m56gs6vrxbyza because a different package
780a2c2f-f4be-4a7d-83bf-212522026da9_1.0.0.0_x64__gf73qhakswkrp with the same name is already installed. Remove package
780a2c2f-f4be-4a7d-83bf-212522026da9_1.0.0.0_x64__gf73qhakswkrp before installing.

…and you might not know how to find the full internal list of installed UAP apps or to remove them. For me, the app in question did not show up in Apps and Features.

How to fix it

Open PowerShell as Admin (Win+X, A)

Run this Get-AppxPackage command, replacing the 780a2c2f with part of the name from your error message: